The control of replication of the genome and the mechanisms for the replication are the subjects of the proposal. Initiation of replication is not understood, but appears to be an active process requiring protein synthesis and energy. We have begun investigating initiation by use of an in vitro system. It appears that normal termination and cessation of synthesis induced by dnats initiaton mutations can both be reversed in vitro under appropriate conditions. Investigation will also be directed towards defining the roles of the several DNA polymerases in the maintenance of the integrity of the chromosome following UV irradiation. Repair synthesis can apparently be catalyzed by different DNA polymerases, but the characteristics differ.